The present invention generally relates to a semiconductor device and associated semiconductor device fabrication method, and, more particularly, to a technology for providing, in the VLSI (very-large-scale integrated circuit) multilevel metallization formation steps, an interlayer dielectric film having an excellent uniformity of film thickness.
As the scale of semiconductor device integration increases and device dimensions decreases, demand for sophisticated multilevel metallization technology becomes higher and higher. The interlayer dielectric film serving as an insulator therefore plays an important role to provide a conformal step coverage to the walls and bottom of a fine step of a first metallization layer having a high aspect ratio, and, recently, an atmospheric-pressure chemical vapor deposition (APCVD) method utilizing the reaction of tetra-ethyl-ortho-silicate (TEOS) with ozone (O.sub.3) has been in intensive research, since TEOS deposition provides a good step coverage.
Referring now to the accompanying drawings, a conventional semiconductor device fabrication process is described. FIGS. 11a-c show subsequent steps of the fabrication of a conventional semiconductor device. Formed on semiconductor substrate 10 are LOCOS film 11 for insulation/separation, polysilicon gate electrode 12, boron oxide/phosphorus oxide-containing SiO.sub.2 layer 13 (hereinafter called "BPSG layer"), source (or drain) region 14 that is an impurity-diffused region, tungsten source (or drain) electrode 15, and first metallization layer 16 that is an aluminum alloy layer having a content of about 1% silicon and about 0.5% capper. In addition, gate insulating layer 17 is shown in FIG. 11c.
SiO.sub.2 layer 21 (hereinafter called "P-TEOS layer") is first deposited on first metallization layer 16 by means of a plasma CVD process from a TEOS source gas (see FIG. 11b).
Next, SiO.sub.2 layer 22 (hereinafter called "TEOS-O.sub.3 layer") is deposited by means of an APCVD process making use of the reaction of TEOS with O.sub.3. This is followed by the formation of second metallization layer 23 on TEOS-O.sub.3 layer 22 (see FIG. 11c).
The above-described APCVD utilizing the reaction of TEOS with ozone much depends on the underlying topography. Suppose that an underlaying layer, or P-TEOS layer 21 has different types of film properties, namely a hydrophilic (water-loving) region and a hydrophobic (water-hating) region and TEOS-O.sub.3 layer 22 is deposited on such P-TEOS layer 21. In this case, TEOS-O.sub.3 layer 22 suffers some problems such as the differences in the formation rate and film property between a part of TEOS-O.sub.3 layer 22 overlying the hydrophilic region of P-TEOS layer 21 and another part overlying the hydrophobic region. These problems are described below in detail.
FIG. 12 theoretically depicts the profile of P-TEOS layer 21 deposited on first metallization layer 16. As seen from FIG. 12, P-TEOS layer 21 has a project part where first metallization layer 16 underlies and a recess part between first metallization layers 16. The project part, because it stands tall, is exposed intensively to ion species and electrons when a plasma CVD process is being performed. As a result, good inter-atom bonding between Si and O occurs in such a projecting surface layer, thereby producing a surface layer state having a less number of dangling bonds or "arms". Generally, dangling bonds join to hydroxyl groups (--OH), in air. In contrast, the recess part is less exposed to ion species and electrons as compared with the project part, thereby creating a greater number of dangling bonds which bond to "--OH" groups upon exposure to air. To sum up, if P-TEOS layer 21 has different types of surface states, this makes the film thickness of TEOS-O.sub.3 layer 22 vary.
The conventional semiconductor device fabrication process has several drawbacks. Because of the difference in material between P-TEOS layer 21 and TEOS-O.sub.3 layer 22, or because of the concurrent existence of a hydrophilic region and a hydrophobic region in an underlying layer, the film formation rate and film properties of TEOS-O.sub.3 layer 22 vary. As a result, the surface of TEOS-O.sub.3 layer 22 becomes uneven, and its surface property gets worse.
There have been proposed techniques of exposing P-TEOS layer 21 to a N.sub.2 -gas plasma, and some of them are shown in the following literature.
1) K. Fujino, Y. Nishimoto, N. Tokumasu, and K. Maeda, "Surface Modification of Base Materials for TEOS/O.sub.3 Atmospheric Pressure Chemical Vapor Deposition," J. Electrochem. Soc., Vol. 139, No. 6 (1992) p. 1690. PA0 2) N. Sato, Y. Ohta, T. Hashimoto, A. Kotani, and M. Ishihara, "TEOS-O.sub.3 APCVD SiO.sub.2 Dependence Upon Underlying Topography," '92 39th Applied Physics Society Spring Meeting Draft Collection, 29a-ZG-4, p. 646. PA0 3) H. Kotani, and Y. Matsui, "Organo-Silicon CVD technology," Semiconductor Research 36, p. 18. PA0 4) H. Kotani, M. Matsumura, A. Fujii, H. Genjou, and Nagano, "Low-Temperature APCVD Oxide Using TEOS-Ozone Chemistry for Multilevel Interconnections," Tech. Dig. International Electron Devices Meeting (1989) p. 669. PA0 5) Y. Nishimoto, and K. Maeda, "Reduction in Hygroscopic of TEOS-O.sub.3 CVD Film After Plasma Process", Semiconductor World February Issue (1993), p.82. PA0 6) Y. Hosoda, H. Harada, A. Shimizu, K. Watanabe, and H. Ashida, "Low Temperature Anneal Effect On Hygroscopic of TEOS-O.sub.3 APCVD NSG Film" Semiconductor World, February Issue (1993), p. 77.
It can be taken that, when P-TEOS layer 21 is exposed to a N.sub.2 -gas plasma, it (i.e., P-TEOS layer 21) takes either one of two types of surface forms described below.
FIG. 13 illustrates one of the two surface forms, and the FIG. 14 shows the other surface form. In the former surface form, "Si--O--Si" bonds or "Si--OH" bonds are broken upon exposure to a N.sub.2 -gas plasma, and the number of dangling bonds, shown in the figure in the form of "Si--" or "Si--O--", increases. In the latter surface form, silicon joins to nitrogen to produce "Si--N--Si" bonds, and the number of dangling bonds decreases.
In both the above-described cases, when a N.sub.2 -gas plasm process is performed, N ions or electrons "fly" to the surface of P-TEOS layer 21, with taking an arriving angle, .theta..sub.1, with respect to the project part and an arriving angel, .theta..sub.2 with respect to the recess part (see FIGS. 13 and 14). Due to the difference between the arriving angle .theta..sub.1 and the arriving angle .theta..sub.2, the project part and the recess part receive different amounts of ions or electrons. As seen from FIGS. 13 and 14, the arriving angle .theta..sub.1, is about 180 degrees, whereas the arriving angle .theta..sub.2 is small. Additionally, the value of the arriving angle .theta..sub.2 varies with the recess part pattern. The N.sub.2 -gas plasma process is dependent on the form of an underlying layer (i.e., first metallization layer 16), and it is difficult to control the surface state of P-TEOS layer 21.
The N.sub.2 -gas plasma process presents the problem in that P-TEOS layer 21 is formed into a stepped profile depending on the presence or absence of first metallization layer 16, and the effect of the N.sub.2 -gas plasma process differs depending on the stepped profile of P-TEOS layer 21.
Further, the N.sub.2 -gas plasma process always requires P-TEOS layer 21 as an underlying layer and an apparatus for performing a N.sub.2 -gas plasma process, which complicates not only processing requirements but also processing steps.